Mechanisms are mechanical structures synthesized with assemblages of joints and links designed to provide them with predictable structural, kinematic and dynamic properties. They are the basis for vast numbers of applications including cars, aircraft, optical instruments, manipulation devices, etc. and as such are particularly important elements of most technological systems.
Mechanisms are synthesized by constraining joints or articulations to fixed relationships by means of links. A kinematic analysis assumes links to be ideally rigid. Most mechanisms can be described by selecting one output link and one ground link and defining the elements there-between.
Parallel mechanisms, a vast sub-class of all mechanisms, offer an opportunity for improved structural properties with rigidity, light weight and improved dynamic properties. Parallel mechanisms used in drives allow actuators to be placed at locations where they contribute the least to an increase of inertia. Further, improved accuracy can be achieved by eliminating the accumulation of errors.
Unfortunately, most known parallel mechanisms with more than two or three degrees of freedom suffer from a reduced usable workspace. The invention reported herein achieves a significant improvement in this area.
If a chain of links and joints forms loops, then the mechanism is termed parallel. If a mechanism requires exact geometrical properties to possess mobility (degrees of freedom), it is termed over-constrained. If a mechanism has no mobility, it is called a `structure`. (The term `structure` may apply to other notions but should be clear by context). If there are no loops the mechanism is called serial.
As joints play a central role in mechanisms and are needed to describe the invention, they are defined herein. The joints needed to describe the invention belong to the class of lower pairs because they can be defined by specifying certain pairs of surfaces which have the property of allowing relative motion without the surface contact being lost. Two surfaces of revolution form the revolute joint which has one angular degree of freedom. Two cylindrical surfaces define the cylindrical joint which has two degrees of freedom, one angular about the axis of the cylinder and one translational along the same axis. Two surfaces shaped as parallel prisms form the prismatic joint which has one freedom of translational motion. The modes of realization of these basic joints include a variety of techniques, e.g. rolling elements of locally deforming members, but these do not change the definition.
A "universal" joint is composed of two non-collinear, preferably orthogonal revolute joints with a center of rotation at the intersection of their axes.
The spherical joint has three degrees of angular freedom of motion. A spherical joint may be composed of spherical surfaces in contact, vis, a ball-in-socket; or can be created by three orthogonal revolute joints with a center of rotation at the intersection of their axes.
A "gimbal" joint has three revolute joints positioned to rotate about a common center of rotation.
Actuated joints are equipped to provide mechanical power derived from an external source. Passive joints are left free to move by virtue of the forces present in the links. Joints may be actuated to provide rotational or translational motion.
Any joint can be instrumented with sensors to measure position or velocity of the relative motion of links. Mechanisms can, therefore, be reversed in their functions. Rather than controlling a driven link through actuators placed at joints, the actuators may be replaced by sensors which detect the position and/or orientation of the former "driven" but now "sensing" link. In such cases it is particularly important for the mechanism to have reduced inertia in order to permit it to track higher frequency oscillations to which the sensing link may be exposed. Mechanisms which are both instrumented and actuated are amenable to feedback control as applied to most drive systems.
As a further variant on the application of such mechanisms, actuators may be replaced by locking devices such as "brakes". In this configuration, a mechanism can be positioned to provide support as a jig, and then become locked in place. In such application, it is desirable for the locked mechanism to remain precisely positioned after being locked.
Four bar mechanisms having four links and four joints are used in a bewildering number of applications. Many functions can be accomplished by changing the four kinematic design parameters (link lengths). If the axes of joints are not exactly parallel, the "mechanism" becomes a structure with rare exceptions.
In a four-bar planar mechanism, if one link is grounded as a "base" link, then the link opposite the ground link--the output or driven link--can be displaced along a curve in a plane through the manipulation of the links proximate to the base link--the "proximal" links. The driven link may carry an object or a further mechanism, such as a claw, tool or "end effector".
A five-bar mechanism has five links and five joints. If one link is grounded as a "base" link, then the joint opposite the grounded link--the "driven joint" --can be displaced in a plane through the manipulation of the links proximate to the base link--the "proximal" links. The remaining two links next to the driven joint may be classified as "distal links".